Field effect transistors (FETs) are widely used in the electronics industry for switching, amplification, filtering, and other tasks related to both analog and digital electrical signals. Most common among these are metal-oxide-semiconductor field-effect transistors (MOSFETs), wherein a doped polysilicon gate is energized to create an electric field within a semiconductor channel underlying the gate, by which current is allowed to conduct between doped source/drain regions formed in a substrate on either side of the channel. In order to provide a conductive gate electrode, a polysilicon gate structure is patterned over the prospective channel region of the substrate and dopants are added to render the polysilicon conductive. The doping of the polysilicon gate structure is usually performed simultaneously with the doping of the source/drain regions of the substrate, typically through implantation processing. The doped polysilicon gate structure overlies a thin gate dielectric layer formed over the channel substrate.
The gate dielectric is an insulator material, which prevents large currents from flowing from the gate into the channel when a voltage is applied to the gate contact, while allowing such an applied gate voltage to set up an electric field in the channel region in a controllable manner. In operation, the resistivity of the channel may be controlled by the voltage applied to the doped gate structure, by which changing the gate voltage changes the amount of current through the channel. The doped polysilicon gate structure and the channel are separated by the gate dielectric, which is an insulator. Thus, little or no current flows between the gate and the channel. However, the gate dielectric allows the gate voltage to induce an electric field in channel, by which the channel resistance can be controlled by the applied gate voltage.
In the manufacture of such devices, there is a continuing trend toward higher device densities, and hence smaller and smaller device dimensions. Generally, device density is improved by scaling or decreasing the size of the transistors and other electrical components. In this continuing process, it is desirable to provide sufficient polysilicon doping to accommodate the smaller device sizes. In addition, although generally scaled to be smaller, certain devices require larger feature sizes than others, including gate dimensions. Typically, the doping of the polysilicon gate structures is performed in a single implantation step across all the polysilicon gate structures in a semiconductor device.
After the polysilicon is doped, subsequent processing of the semiconductor device may lead to a depletion of dopants in selected regions of the polysilicon (“dopant depletion”). This is typically due to out-diffusion of the dopants into either the ambient or surrounding films during high processing at elevated temperatures. This loss of dopants is proportional to the polysilicon surface area and results in a reduction in the average doping at the polysilicon-gate dielectric interface at the completion of the processing. This condition, referred to as “poly depletion”, causes an increase in the region of polysilicon that is depleted of carriers when the gate is biased to allow accumulation in the MOS channels. The increase in the effective thickness of the gate oxide under the inversion condition has the effect of an increase in threshold voltage and reduction in gate capacitance, in turn causing a reduction in transistor drive current and increased logic gate delay and processing time.
In order to provide process uniformity and control over individual device performance, it is desirable to ensure that the dopant concentrations in all the gate structures be the same in both small and large polysilicon gate structures when the manufacturing process is completed. Accordingly there is a need for processes and methodologies by which poly depletion can be mitigated or controlled in order to reduce the dopant loss and to improve uniformity for end-of-process poly gate dopant concentration across devices having different gate dimensions.